Using GWLF for Development of “Reference Watershed Approach” TMDLs

Total maximum daily loads (TMDLs) are required to be developed in each state for stream segments that have been listed on their 303(d) list of impaired streams. Where a numeric standard has been set for a pollutant, the indicator of a restored condition – the TMDL target load – is readily definable. Where a quantitative standard does not exist for a pollutant, as is the case for sediment and nutrients in many states, an alternative approach is required to define the TMDL target load. One such alternative is the Reference Watershed Approach. This paper describes how the GWLF model has been used with the Reference Watershed Approach to develop TMDLs for benthic impairments in Virginia based on long-term average annual sediment loads. Although developed for use with ungaged watersheds, studies have shown that calibration improves modeled stream flow, where observed data is available. A modification of the GWLF code is, therefore, described that allows output of monthly pollutant loads by land use to enable calibration to monthly model output. This paper further describes post-processing spreadsheet procedures developed to account for point source loading, modeling of sub-watersheds, and reductions due to implemented BMPs. Quantification of the TMDL target load and creation of alternative allocation scenarios required as part of initial TMDL studies are also part of the post-processing procedures.